The invention relates to a process and to an apparatus for direct production of cast iron from iron bearing ore (hereinafter referred to as xe2x80x9ciron orexe2x80x9d) having grains of different sizes and from fine coal.
Other processes for producing cast iron from coke and iron ore after pre-treatment processes like sintering or pelletizing are well-known and experimented and are presently the most common industrial way of producing cast iron. Processes for producing cast iron directly from iron and coal are presently under study.
1. Background Art
In the U.S. Pat. No. 3,607,224 a process is described where the whole conversion from iron ore into molten cast iron is carried out in one reactor, by adding some reagents in tangential direction into the reactor""s top chamber.
In Malgarini et al. xe2x80x9cFirst results from cleansmelt ironmaking pilot plantxe2x80x9dxe2x80x94Iron and Steel Engineer Vol. 74, No1, P.30-31, Jan. 1997 a smelting reduction process is described wherein a reactor including a zone for prereduction and preheating and a zone for final reduction and carburization of liquid metal is described.
EP-690 136 describes a two-stages process for production of iron from iron compounds in a two chamber apparatus.
EP-726 326 describes a two-stages process for the production of pig iron wherein iron ore is directly reduced in a pre-reduction stage followed by a final reduction stage.
2. Disclosure of the Invention
The present invention relates to a process and to an apparatus for the production of molten cast iron in one single reactor from iron ore, fine coal, oxygen and fluxes.
More particularly, the present invention refers to a process for producing cast iron directly from fine iron ore and fine coal in an apparatus comprising one reaction chamber (reactor) only that includes two areas in open communication, one top area and one bottom area, where the process is carried out, this process being characterized by comprising the operations of:
feeding fine iron ore together and simultaneously with oxygen or preferably with a mixture of oxygen and air into the top area of the reactor through its side walls;
feeding fine coal, oxygen, fluxes and a carrier gas into the bottom area of the reactor through its side walls and a stirring gas from its bottom.
The invention provides carrying out the process in one reaction chamber (reactor) where two process stages take place, in two separated areas (hereinafter also referred to as xe2x80x9cthe dresserxe2x80x9d and xe2x80x9cthe converterxe2x80x9d for the top area and the bottom area respectively).
The two process stages are as follows:
in the dresser: iron ore is fed through a plurality of nozzles and carried by a stream of oxygen or preferably by a mixture of oxygen and air. It comes in contact with a hot and reducing gas generated in the bottom area of the reactor (converter), under such conditions that a partial combustion of the gas coming from the converter takes place, which cause a pre-heating and pre-reduction of the iron ore.
The pre-heated and pre-reduced iron ore is fed into the converter. The converter""s performances depend on the chemical and thermal performances that are obtained in the dresser, and in particular on the pre-reduction degree and on the pre-heating temperature of the pre-reduced iron ore.
in the converter: the pre-reduced iron ore is completely reduced and molten cast iron is generated. A fuel, e.g. fine coal, and a supporter of combustion, e.g. oxygen, together with fluxes, a carrier gas and a stirring gas are introduced into the converter, where the hot reducing gas, which is fed into the dresser, is produced together with cast iron.
The dresser is connected to a discharge conduit for discharge of process gas. The process gas leaving the discharge conduit is cooled down by water jets and then crosses a Venturi pipe scrubber, where the particulate that may have formed in the gas is made to settle. Finally the gas is released into the utility network.
The process described can produce cast iron by operating the dresser in such a way so to pre-reduce iron ore at least into Fe3O4, while it is pre-heated to temperatures not lower than 800xc2x0 C.
It is known that the main component of iron ore is hematite (Fe2O3) which can be reduced to Fe3O4 at temperatures that are not very high and in environment of relatively low reducing potential (CO/(CO+CO2) greater than 0.1 and H2/(H2O+H2) greater than 0.1).
As a consequence, pre-reducing iron ore to Fe3O4 is a feasible target and the finer is iron-ore grain, the quicker the kinetics of the reaction. With a gas temperature around 1600xc2x0 C. or higher and iron-ore grain of a few millimeters, pre-heating iron ore to approx. 800xc2x0 C. can take a remarkably short time of around one second of interaction between gas and iron ores.
Preferably the iron ore has a grain size smaller than 8 mm, and has a content of iron at least in part lower than 50%. The velocity of iron ore and oxygen introduction in the reactor top chamber is generally lower than 40 m/s.
The energy required by the chemical reactions and physical transformations taking place in the converter is mainly provided by the post-combustion of the reducing gas with oxygen (the oxygen blown into the converter is hereinafter referred to as xe2x80x9cprimaryxe2x80x9d or xe2x80x9csecondaryxe2x80x9d according to whether it is injected to a lower or higher level in the converter, while the oxygen blown in the dresser can be referred to as xe2x80x9ctertiaryxe2x80x9d). The reducing gas is generated from coal gasification when coal interacts either with (primary) oxygen blown together with coal, or with oxygen associated with the pre-reduced iron ore. The gasification reaction requires an adequate temperature in the system in order to be activated. Coal and (primary) oxygen are injected into a molten slag bath, that is composed of mineral gangue and ashes from coal and fluxes and which floats on the molten cast iron bath throughout the bath formation.
The grain size of fine coal and fine fluxes is preferably smaller than 3 mm. In the converter, iron oxides are reduced into metallic iron when they react with the coal scattered in the slag and with carbon monoxide that generates from coal gasification with primary oxygen: at the same time, iron is carburized resulting in molten cast iron. The chemical and thermal energy required by these reactions is provided by oxygen blown in the slag and by coal injected either as fine coal or added partly as fine coal and partly as sized coal that falls down under its own weight.
Simultaneous injection of coal and oxygen under the liquid slag surface by means of co-axial lances, the coal being carried by an inert gas, creates the appropriate temperature conditions and intimate mixing of coal and oxygen for a rapid coal gasification thus allowing to utilize any kind of coal, from low-volatile to high volatile.
Primary oxygen injection is adjusted so as to keep residual non-gasified coal scattered in the slag bath. It contributes to the final reduction of iron ore into metallic iron and to its carburization into cast iron.
Also secondary oxygen is injected under the slag surface so that heat from post-combustion develops inside the slag.
Heat transfer from slag to cast iron and uniformity of temperature and chemical composition within the slag and the metallic phase are assured efficiently by the convective motions induced by injecting coal and primary and secondary oxygen and by blowing the inert gas from the reactor""s bottom.
Coal and primary oxygen lances are also used to inject basic fluxes (mainly calcium oxide) together with coal and simultaneously to primary oxygen. Preferably the ratio coal to flux ranges from 3 to 15.
Under these conditions, coal gasification occurs in the presence of a high local concentration of calcium oxide that permits on the one side to improve fixing in the slagxe2x80x94in the form of stable compoundsxe2x80x94of sulphur from coal gasification, and on the other side to quickly neutralize silica from the coal ashes.
The process as above described provides for a number of benefits:
the tendency of sulphur to solubilize into cast iron is reduced, thus improving the quality of cast iron;
the quantity of sulphur in the exhaust gas is strongly decreased, resulting in advantages to the environment;
silica from coal ashes is quickly fixed into calcium silicates. This avoids that slag is locally enriched with silica thus decreasing the chemical attack of the refractory material around the injection lances;
the tendency of slag to foam is controlled. It is reminded that slag enriched with free silica tends to foam and that addition of fluxes contributes to generate a slag where silica is neutralized by basic oxides (e.g. CaO and MgO).
While slag foaming promotes heat exchange between the hot gas (gaseous phase of foam) and slag (liquid phase of foam) and also increases heat exchange with metalxe2x80x94thus stating the importance of foamy slag for the processxe2x80x94on the other side an excessive foaming can make the process unstable due to rapid raising of the slag height in the reactor. The best compromise is reached by acting on the slag composition in such a manner to obtain a binary basicity index, IB2, defined as the ratio [%CaO]/[% SiO2] in the range of 1.1 to 1.3.
The above modalities of addition of reagents, i.e. blowing a stirring gas from the reactor""s bottom and combined side-lance injection of coal, fluxes and primary oxygen from bottom lances and secondary oxygen from top lances below the slag surface, give rise to a high reaction velocity and to an efficient heat exchange between the various phases (molten slag, molten cast iron and process gas) in the converter.
In addition, this configuration guarantees high flexibility of the values of post-combustion which can be achieved.
As a consequence, the apparatus described by the present invention can guarantee a high operating flexibility and high specific productivity.
The stirring gas introduced in the reactors bottom has preferably a power lower than 2.5 kW per ton of molten bath.
The invention also provides an apparatus for direct production of cast iron from iron ore and sea coal characterized in that it encompasses a reaction chamber (reactor) having:
a top area of a substantially rotationally symmetrical shape;
means for supplying iron ore, oxygen or a mixture of oxygen and air to said top area;
a discharge conduit connected with the top area, possibly shaped as two truncated cones with the larger bases in common, in case separated by a substantially cylindrical connector,
a bottom area arranged beneath the top area, of a substantially rotationally symmetrical shape connected to the top area through a connector possibly shaped as a truncated cone;
means for supplying oxygen to said bottom area;
means for supplying oxygen, fine coal, fine fluxes and a carrier gas to said bottom area;
means for supplying a stirring gas to the bottom area.